Power transmission mechanisms that can be implemented between a drive machine and a gear component in drive trains, particularly for mobile applications, and even more particularly for motor vehicles, are known in a multitude of versions from the existing art. These generally comprise an input and at least one output, where the input is coupleable with the drive machine directly or indirectly through additional transmission elements, and at least one output, which is connected to a gear component positioned after the power transmission mechanism, normally a manual gear-changing unit or a continuously variable transmission, and is formed by a transmission input shaft. Situated between the input and the output is a hydrodynamic component, preferably in the form of a hydrodynamic speed variator/torque converter. The latter includes at least one primary wheel that functions as a pump wheel when power is being transmitted from the input to the output, and one secondary wheel that functions in this power transmission direction as a turbine wheel. When executed as a hydrodynamic speed variator/torque converter, at least one guide wheel is also provided.
To bridge over the hydrodynamic component, a device is provided that is also known as a lockup clutch. The latter is normally designed as a switchable clutch operating on the principle of friction, and includes a first clutch part and a second clutch part, which may be brought into operative connection with each other, at least indirectly. The lockup clutch serves here as a coupling between the input and the output, in particular as the coupling in the connection between the input and pump wheel with the turbine wheel, or the connection between turbine wheel and output. Actuation of the switchable clutch is accomplished by way of a control device, which in its simplest form comprises an actuating device in the form of a piston element chargeable with a pressurizing agent.
If the power transmission mechanism is designed as a three-channel unit, it includes at least three connections: a first connection which is coupled with the working chamber of the hydrodynamic component; a second connection which is coupled with the interior of the power transmission mechanism; and, a third connection which is coupleable with a chamber that is chargeable with a pressurizing agent, which chamber is assigned to the actuating device and through which the pressure in the actuating chamber is freely adjustable.
In this design of three-channel construction, the control device is subjected to a separately controllable pressure. The flow-through direction of the hydrodynamic component is controlled through the other connections to the working chamber of the hydrodynamic component and the intermediate spaces between the hydrodynamic component and the lockup clutch or the control device. The flow-through is then either centripetal or centrifugal, depending on the mode of operation or the actuation of the connections in a pressurizing agent supply system to achieve a circuit that is in operation externally to the flow circuit that develops in the hydrodynamic speed variator/torque converter and is routed either within the power transmission mechanism or externally.
Power transmission mechanisms of this type are normally operated in two different operating modes, which differ in the flow of force through two different power branches; overlapping operation in both power branches is also possible. There the transmission of power in a first power branch takes place via the hydrodynamic component. In this case the lockup clutch is deactivated and the pump wheel is coupled with the input, while the turbine wheel is connected to the output in a rotationally fixed connection. For bridging, the lockup clutch is activated and the hydrodynamic component is removed from the power stream. However, that is normally accompanied by a strong engagement impact, which is caused in part by the non-equilibrium of the centrifugal oil pressures on the piston element of the control device for the lockup clutch.
Furthermore, the switchable clutch device can be closed actively by pressurizing the pressure chamber assigned to the control device, which is normally realized by a filling pulse in the hydraulics. This causes a relatively high volume flow, whereby the control device, in particular the piston element, is brought beyond the air space to bear against the individual clutch parts, in particular in the form of lamellae, and the necessary torque for the transmission is built up. At the same time it is necessary, however, that the filling pulse be controlled so that the flow volume becomes zero exactly at the moment when the piston element, i.e., the control device, touches the clutch parts, in particular lamellae. Otherwise there will be a strong pressure rise, which is manifested in an engagement impact.
In another case the control device is not in contact, and the engagement impact follows when an attempt is made to build up pressure on the converter clutch. This system is very sensitive to influences of tolerances and the environment, in particular friction, temperature and the centrifugal oil pressures, and thus is dependent on parameters that change during operation. The bridging clutch is path-controlled in the transition from the disengaged state to the power transmitting state, and is power-controlled in the power transmitting state. A disadvantage of the systems known heretofore is in particular the engagement impact, which has a negative impact on the driving behavior and also promotes wear. Such a design of a power transmission mechanism is described by way of example in published patent DE 103 52 963 A1.